1. Field of the Invention
The present invention relates to a semiconductor device such as a thin film transistor (hereinafter, referred to as TFT) which is used in, for example, a liquid crystal display device, and a method for producing the same.
2. Description of the Prior Art
A liquid crystal display device, which has features of being thin and consuming low power, has been a focus of attention as a novel device for replacing a CRT (cathode ray tube). Technological development has been especially active in the field of an active matrix liquid crystal display device including a TFT array, which has advantages such as high responding speed and high display quality. The target for further developments of this type of liquid crystal display devices includes reduction of the size of the display pixel to attain a higher precision, enlargement of the display area to realize a larger display screen, and improvement of the resistance against chemicals to facilitate the production process.
For achieving the above-mentioned specific developments, a gate electrode wiring and a source electrode wiring of a TFT, which are provided in the vicinity of a pixel electrode, are required to be made thinner and longer and to be highly resistant against the chemicals used in the production process. In the case when an active matrix substrate includes a reverse stagger TFT disposed on an insulating substrate, for example, Ta has conventionally been used for the gate electrode wiring to fulfill such requirements.
It is known that a thin Ta film has a crystalline structure of .beta.-Ta and has an extremely high specific resistance of 170 to 200 .mu..OMEGA..multidot.cm if it is formed by use of a conventional sputtering method or the like.
It is also known that a thin Ta film has a crystalline structure of .alpha.-Ta and has an extremely low specific resistance of 40 to 70 .mu..OMEGA..multidot.cm if it is formed by use of sputtering in an Ar gas including a small amount of N.sub.2. Accordingly, a thin film of .alpha.-Ta obtained in this manner has been used for an electrode wiring of a liquid crystal display device having a large display screen, for example, of approximately 10 inches. However, the thin film of .alpha.-Ta has a problem in that the specific resistance is unstable.
Japanese Laid-Open Patent Publication No. 62-205656 discloses a semiconductor device having an electrode wiring formed of an alloy of Mo and Ta. This electrode wiring also has a problem in that Mo is eluted when the electrode wiring is anodized, resulting in insufficient insulation of the anodized film.
A semiconductor including an electrode wiring which has a two-layer structure is also known. For example, Japanese Laid-Open Patent Publication No. 2-106723 discloses a TFT array including a two-layer electrode wiring which has a lower layer of Nb formed on an insulating substrate and an upper layer of Ta formed on the lower layer. Such an electrode wiring has a problem in that the lower layer can be etched too much to allow the upper layer to overhang the lower layer, thereby making the protection thereof by an insulating film difficult.
Thin Solid Films, 16, (1973) pp. 129-145 discloses a two-layer film including a lower layer, closer to a substrate, and an upper layer. When the lower layer is formed of Au, Pt, Rh, Be, W, .alpha.-Ta, Mo, Si.sub.3 N.sub.4, or Ta.sub.2 N, the upper layer formed of Ta has a crystalline structure of .alpha.-Ta which is low in specific resistance. However, the lower layer as thick as 500 nm causes a problem in that, when such a two-layer film is applied to an electrode wiring, the electrode wiring cannot sufficiently be coated with a gate insulating film. As a result, a shortcircuit may occur between the gate electrode wiring and a source electrode wiring, or the gate electrode wiring may be disconnected. The above two-layer film has a further problem that the upper layer and the lower layer cannot be etched by an identical type of etchant.